88 Mercury fuel problem

[NCfsa99]

I am not an outboard mechanic and this has probably been hashed out before, but I would appreciate your suggestions and advise. I have an '88 Mercury 35hp outboard. Just had it rebuilt by a certified Johnson mechanic, this may be the problem Anyway the motor starts good and runs in bursts, length or run depends on speed. The same mechanic assured me that the problem was in the fuel pump, so he put a repair kit in it. Same problem motor runs out fuel in carb. primer bulb goes soft engine cuts off. The engine will continue to run if you keep pumping up the bulb. So then he has me replace the fuel line and primer bulb, same problem. Now he says to replace the tank. I know that he is eleminating possiblilities but I want to fish! I read an old post about a Johnson with a seperate oil mixer unit that had been removed, they said they had to put a different type fuel pump on it. My motor originally had a remote oil mixing unit also but the mechanic removed it and recommended mixing oil in tank after the rebuild. Do I need to replace fuel pump with a different kind?

 

[Trent]

Re: 88 Mercury fuel problem

No, You dont need a new pump. Iv done this several times. Check the fuel pump. When he removed the oil tank he should have "plugged" the port on the fuel pump where it will not suck air. If he did; did it come loose?<br /><br />Also I have seen bad check valves in the pump do the same problem as you have. Did he replace the check valves?? Not all rebuild kits come with check valves.

 

[NCfsa99]

Re: 88 Mercury fuel problem

Thanks for the reply Trent, I watched as he put the rebuild kit in, he replaced the two large diaphrams and gaskets and the two small round parts (check valves?). The oil mixing unit was a seperate part and was connected by the gas line and 3 electrical wires. What should have been pluged?

 

[Down South]

Re: 88 Mercury fuel problem

Is the primer bulb collapsing or just going soft? If it's collapsing your tank vent is either not open or plugged, or the pick up tube in your tank is restricted. If the bulb is just going soft then you or either sucking air or the mechanic botched the job on the fuel pump.If the bulb is just going soft remove the cover on the motor and pressure up the fuel system with the primer bulb. Check all of the fuel line from the primer bulb to the fuel pump for fuel leaking out. If you spot a leak repair it.

 

[Trent]

Re: 88 Mercury fuel problem

The oil pump (mixing unit) should be disconnected. The fuel line runs to the fuel pump. Remove the line and plug it where it goes onto the fuel pump and plug where it went onto the oil pump. You can buy rubber "vacuum" plugs at the autoparts store. I personally remove the oil pump and take out the gears and then reinstall the pump.<br /><br />You also need to check the fuel tank vent. Try running the engine with the cap off and see if that helps.

 

[Down South]

Re: 88 Mercury fuel problem

NCfsa99, Somehow I missed the part about the oil tank being removed. Take Trent's suggestion and check that port. This is a strong possibility of what may be causing the problem.

 

[NCfsa99]

Re: 88 Mercury fuel problem

Thanks guys, but I must be missing part of what you are telling me, I'll try to catch up here. My oil mixing unit was seperate from the boat motor. The gas line went as follows: tank, primer bulb, oil mixing unit,gas line connected externally to motor. Now the oil unit was taken out completely and replaced with a new gas line w/primer bulb. The fuel pump had three commections: lower foward port marked in, upper rear port marked out and a port on the outside unmarked and I didn't notice where that line went. Down South yes the primer bulb goes soft. I will look this evening to see if I can see any leaks. I really appreciate all of you imputs I think we're getting somewhere now.